1. Field of the Invention
This invention relates to a catalyst useful in a process for the polymerization of 1-olefins having a maximum of 8 carbon atoms in the chain and having no branching nearer the double bond than the 4-position. The catalyst is formed on a silica xerogel support having a cummulative pore volume, pore diameter distribution, and surface area such that relatively low molecular weight, high melt index polyolefins are readily produced in particle form polymerization carried out therewith.
2. The Prior Art
Silica gels find numerous applications, chief amongst which are as adsorbents and catalyst supports. The latter application in particular has attracted increasing attention in recent years, especially in connection with catalysts for the stereospecific polymerization of olefins. Catalysts having sterospecific activity include metal-containing catalytic materials, e.g. chromium supports which have previously been activated by oxidation at elevated temperatures. Olefins may be polymerized with with such catalysts to produce a varied series of polymers having differing molecular weights and melt indexes, depending upon the particular temperatures, pressures, solvents or other diluents, catalysts or other reaction conditions used.
For many applications the production of low molecular weight, high melt index polymers is of particular advantage, such materials finding important applications in films and sheets, extrusion coating, injection and rotational molding, the like. Considering the preparation of ethylene polymers as illustrative, low molecular weight (high melt index) polyethylenes are commercially obtained by carrying out the polymerization in solution (the "solution process"), but only at relatively low conversion, measured as pounds of polymer per pound of catalyst. When the reaction is carried out in suspension (the "suspension" or "particle form" process), it is possible to obtain higher conversions. The particle form process thus exhibits distinct commercial advantages relative to the solution process for the stereospecific polymerization of olefins. However, particle form slurry operations have been limited, at high conversion rates, to the production of polyolefins having melt indexes lower than about 2. Various techniques have been proposed to increase the melt indexes of olefin polymers so produced, with varying degrees of success. For example, while the use of modifiers such as hydrogen has been found to decrease the molecular weight and increase the melt index of the polymer product, the advantages attendant the use of such materials are limited since they simultaneously decrease catalyst activity. Similarly, variation of the chromium oxide content of the catalyst, addition of different metal oxide promoters, combination of different supports and/or the use of varying catalyst activation temperatures, have been widely investigated, with only marginal improvement.
Modification of the porosity, surface area and other characteristics of the catalyst support has also been suggested as a means for increasing the melt index of olefin polymers produced by particle form stereospecific polymerization reactions. Thus, in recent years a number of procedures have been described in the literature for the preparation of silica gel materials said to be useful as catalyst supports for this purpose. Such procedures are described, for example, in U.S. Pat. Nos. 3,132,125 and 3,225,023; and in British Pat. Nos. 1,007,722. Silica gels so prepared have not, however, achieved their intended purpose, i.e., the production of olefin polymers having markedly increased melt indexes.
Thus, for example Schwander et al U.S. Pat. No. 3,132,125 describes the use in both solution and suspension processes of stereospecific catalysts supported on non-porous silicas for the production of polyolefins said to have relatively low average molecular weights and, correspondingly, high melt indexes. Relatively high melt index polymers were in fact produced in the solution phase operations exemplified by Schwander et al. Where, however, particle form operations were utilized use of the catalyst described in this patent resulted in the preparation of polymer products having melt indexes (estimated from the molecular weight data set forth by Schwander et al) no greater than about 0.2.
Hogan et al U.S. Pat. No. 3,225,023, assigned to Phillips Petroleum Company, suggests that olefin polymers having increased melt indexes may be produced employing catalyst supports having increased average pore diameters, ranging from about 60 to 400 A. Hogan et al illustrate their process by experimental runs (which may have been been conducted in either the solution or suspension phases), employing "commercial silica gel" supports having varying average pore diameters. The use of silica gels of the type commercially available as of the Hogan et al. filing date (November, 1962) and having the indicated range of average pore diameters has not, however, resulted in the formation of very high melt index polymers employing particle form operations. Thus, polyethylene so produced (employing chromium oxide catalysts deposited on such supports) have melt indexes of only up to about 3.0.
British Pat. No. 1,007,722, also assigned to Phillips Petroleum Company, describes the use of a "specific form of high purity finely divided porous silica gel" as a support for a chromium oxide catalyst said to be capable of producing relatively high melt index polyethylene in a particle form polymerization. The specific form of silica gel referred to in the British specification is a silica aerogel having a pore diameter between approximately 200 A and 500 A, a surface area of approximately 250 to 350 m.sup.2 /g, a density of less than approximately 0.2 g/ml, and an oil adsorption of approximately 300 lbs/100 lbs. "Syloid" 244 (having a surface diameter of 350 A is the sole such material exemplified.
Aerogels are silica gels in wich the liquid phase has been replaced by a gaseous phase in such a way as to avoid shrinkage as occurs by direct evaporation of the liquid phase thereof (materials prepared in the latter manner being termed xerogels); Iler, The Colloid Chemistry of Silica and Silicates, Cornell University Press, pages 137 and 152. Aerogels are, however, subject to subsequent strinkage when wetted due to coalescence of their ultimate particles. Shrinkage of this nature decreases porosity and markedly impairs the use of these materials as sterospecific catalyst supports. Moreover, aerogels readily disintegrate when subjected to mechanical stress. Thus, it has been found that the use of silica aerogels as catalyst supports in the particle form process is less than satisfactory.
Nor have other recently disclosed silica gel materials having varying porosity and surface area characteristics proved adequate to effect the production of high melt index olefin polymers in particle form operations. Such materials are disclosed, for example, in U.S. Pat. Nos. 2,731,326; 3,403,109; 3,428,425; and 3,669,624; and in British Patent No. 1,077,908.
As illustrative, Hyde U.S. Pat. No. 3,453,077, and British Patent No. 1,077,908, both of which are assigned to W. R. Grace and Co., disclose methods said to result in the preparation of "microspheroidal silica gels" having pore volumes within the range of from as low as 0.3 cc/g (the British specification) to as much as 2.5 cc/g (the U.S. patent), and surface areas within the range of from 100 to 800 m.sup.2 /g. These references describe procedures for the preparation of silica gels involving gelling alkali metal silicate solutions with gaseous carbon dioxide or mineral acids, neutralizing either about half (the British specification) or substantially the entire alkali metal silicate content of the hydrogels thus formed, aging the neutralized gels (and, in the case of the U.S. patent, making the gel pH alkaline with ammonium hydroxide), thereafter spray-drying the hydrogel to remove the liquid phase, washing the spray-dried material and re-drying the same for subsequent use. It has, however, been found that these procedures do not enable one to prepare silica gel materials having cumulative pore volumes as large as 2.0 cc/g. Moreover, when silica gels thus made are used as supports for stereospecific catalysts in the particle form polymerization of ethylene, polyethylenes having melt indexes of only up to about 2 are obtained.
A major advance in this art was the discovery of techniques for manufacture of silica xerogels having uniquely advantageous physical properties to serve as support for metals as olefin polymerization catalysts. That catalyst is a silica xerogel support having a metal-containing catalytic material deposited thereon. The silica xerogel has a pore volume greater than 1.96 cc/g, e.g., greater than about 2.0 cc/g, the major portion of which pore volume is provided by pores having average pore diameters within the range of from about 300 to 600 A; and a surface area within the range of from about 200 to 500 m.sup.2 /g. The pore volume of the xerogel is suitably provided by pores having a narrow pore diameter distribution primarily within the indicated 300 to 600 A range. The metal-containing catalytic material deposited on the support is preferably a metal oxide, especially chromium oxide or another metal oxide such as cobalt, nickel, vanadium, molybdenum or tungsten oxides. It has been found that a stereospecific catalyst comprising the specified silica xerogel support having the indicated cumulative pore volume, average pore diameter and surface area characteristics is quite effective in particle form olefin polymerizations in producing olefin polymers having markedly higher melt indexes than heretofore obtained.
The polymerization process in which such catalyst is useful involves contacting a 1-olefin having a maximum of 8 carbon atoms in the chain and no branching nearer the double bond than the 4-position (preferably ethylene) with the catalyst under polymerization conditions to provide the indicated high melt index, low molecular weight polyolefin products.
For example, employing that new catalyst, polyethylenes may be readily produced with melt indexes in excess of 2.0, and up to about 15.
Preferably, the silica xerogels employed in the catalyst have cumulative pore volumes ranging from about 2.0 to 2.5 cc/g, with about 70% or more of the pore volume being provided by pores having an average pore diameter within the approximate 300 to 600 A range. Use of such materials as supports for chromium oxide-containing catalysts, for example, results in the formation of polyethylenes having particularly advantageous, high melt indexes ranging from about 3 to 12.5.
The silica xerogel supports of the catalyst, and particularly the porosity and surface area characteristics thereof, are described in terms of their pore volumes (PV), surface areas (SA), and average pore diameters (PD). The surface area is determined by the standard BET method described to Brunauer, Emmett and Teller, J.Am. Chem. Soc., 60, 309 (1938). The pore volume is determined by the well known nitrogen adsorption-desorption technique described, for example, in Catalysis, Vol. II, pages 111-116, Emmett, P. H., Reinhold Publishing Corp., New York, N.Y., 1955 (Run to a P/Po of 0.967 which is equivalent to 600 A, pore diameter) and elsewhere. The pore volumes referred to herein refer to the gel volumes determined by permitting nitrogen gas to be adsorbed by and condensed in the pores of the gel at the normal boiling point of liquid nitrogen and at some relative pressure P/Po, wherein P is the pressure of the nitrogen vapor over the gel and Po is the vapor pressure of liquid nitrogen. For silica gels, the determination of this nitrogen pore volume at a relative pressure P/Po=0.967 permits computation of the volume of those pores having diameters of up to 600 A, which principally contribute to the gel surface phenomena. The average pore diameter may be calculated from this data as follows: ##EQU1##
It should further be understood that, as used herein, "pore volume" and "cummulative pore volume" are synonymous, and refer to the total volume of the pores which comprise the xerogel structure per unit weight thereof. Similarly, the terms average or mean "pore diameter" or "pore size" are used interchangeably herein, and refer to a one-point representation of an actual distribution calculated by the above formula which is based on the geometric model of a right circular cylinder.
The silica xerogels employed in the new supported catalyst are prepared in accordance with the methods disclosed in U.S. Pat. Nos. 3,652,214; 3,652,215; 3,652,216; 3,794,712; 3,794,713; and 3,801,705. The methods described in the noted prior disclosures (which are incorporated herein by this reference) involve the following steps for preparation of silica xerogel;
1. Precipitating a silica hydrogel, under conditions of good-solution, by neutralizing an aqueous alkaline silicon solution, e.g. with a strong acid, a weak acid such as CO.sub.2, an ion exchange resin, or by other suitable means to produce a silica hydrogel slurry, employing the following conditions:
a. the neutralizing medium is added to the aqueous alkaline silicate solution at a rate such that the gel point of the solution is reached in from about 30-120 minutes, e.g. at a rate of up to 40% of the needed amount in 30-120 minutes and the remaining 60% in from about 20-90 minutes more, PA1 b. the temperature during precipitation is maintained between about 0.degree. and 17.degree. C., PA1 c. the SiO.sub.2 concentration in the final slurry is between about 5 and 12% by weight, and PA1 d. the final pH of the hydrogel slurry is from pH 3-8;
2. Maintaining the hydrogel slurry at a pH within the range of pH 3-8 at a temperature and for a time sufficient to strengthen the hydrogel structure;
3. Reducing the concentration of the alkaline material in the hydrogel by washing the same with a liquid which displaces the alkaline material, until the wash liquor recovered contains less than about 20 ppm of the alkaline material, expressed as salt thereof; and
4. Drying the resulting product, either by vacuum freeze-drying (specifically as described in the aforesaid U.S. Pat. No. 3,652,214), solvent displacement (specifically as described in the aforesaid U.S. Pat. No. 3,652,215), or aqeotropic distillation (specifically as described in the aforesaid U.S. Pat. No. 3,652,126). See also U.S. Pat. No. 4,053,565.
The metal-containing catalyst material can be deposited on the silica xerogel support thus formed in any suitable manner. For example, the silica xerogel base can be coated with a metal oxide by shear mixing the finely ground metal oxide with the silica base at room temperature. When chromium oxide is selected as the metal-containing catalyst, 0.5% to 5% by weight and preferably from 1% to 3% by weight based on the total weight of the supported catalyst, may be deposited on the xerogel support. Similar results can be obtained by blending the metal oxide and the carrier under vacuum conditions and/or under a nitrogen atmosphere at 200.degree. C. Activation of the catalyst is carried out in dry air in a fluidized bed at a temperature between about 1500.degree. and 2000.degree. F., and preferably at about 1825.degree. F. The period set for activation is on the order of from 2 to 10 hours and preferably about 6 hours at the foregoing temperatures conditions. The activation is accomplished without any physical change in the carrier.
The effect of these preparational steps is to set the silica gel structure at a desired relationship while in the hydrogel state and thereafter removing the liquid phase of the gel to form the xerogel under conditions to avoid shrinkage by the effect of water. If the hydrogel be dried by simple evaporation, shrinkage occurs, probably by the effect of the water meniscus retreating in the pore. After removal of the liquid phase by techniques described in the patents cited above, the resultant xerogel is impaired as to catalytic properties upon contact with water. The xerogel must be protected against contact with liquid water or moisture content of gaseous media with which it is contacted. For example, the high temperature activation is conducted in a fluidized bed with air which has been very thoroughly dried. This has precluded the use of aqueous solutions for impregnation of the xerogel with a decomposable compound of the metal to be applied. Instead, the water free methods described above are normally employed.
One method of impregnation in the absence of water is described in U.S. Pat. No. 2,734,874. That patent notes the disadvantages of using aqueous solutions and proposes instead a solution of a metal acetylacetonate in an organic solvent. That method uses expensive materials to achieve effective impregnation of a xerogel in the absence of water.